1. Field of the Invention
The present invention relates generally to compensation of a rod position indication system of a nuclear reactor vessel which system includes a linear variable differential transformer (LVDT) having a generally linear output that is representative of the position of a control rod and, more particularly, to compensating such a rod position indication system for any non-linearity in the LVDT output.
2. Description of the Related Art
In a commercial nuclear reactor, heat, from which steam and ultimately electricity are generated, is produced by fissioning of a fissible material such as enriched uranium. This fissible material, or nuclear fuel, is typically contained within a nuclear core made up of a multiplicity of fuel rods supported in a plurality of nuclear fuel assemblies, coextensively arranged in a spaced parallel array.
Movable control rods are dispersed throughout the core to control the fission process. The control rods generally comprise a plurality of elongated rods containing neutron absorbing materials which fit in longitudinal openings defined in the fuel assemblies and among the fuel rods by guide thimbles of the fuel assemblies. The guide thimbles thus guide the control rods during their movement into and out of the core. Inserting a control rod into the core adds more absorber material and, hence, decreases the nuclear reaction; conversely, withdrawing a control rod removes absorber material and, hence, increases a nuclear reaction and thereby the power output of the core. The nuclear reactor core and the control rods are positioned within and supported by a reactor vessel through which a reactor coolant flows.
The control rods are supported in cluster assemblies moved into and from the nuclear core by control rod drive mechanisms which, in turn, are mounted by an upper internals arrangement located within the nuclear reactor vessel above the nuclear core. Typically, a reactor pressure vessel is pressurized to a relatively high internal pressure. The control rod drive mechanisms operate within the same pressure environment that exists within the reactor pressure vessel. Hence, the control rod drive mechanisms are housed within pressure housings of the upper internals arrangement which are tubular extensions of the reactor pressure vessel.
One of the more commonly used types of control rod drive mechanisms is referred to as a "magnetic jack." With this type of mechanism, the control rods are jacked into and from the nuclear core in a series of motions each involving moving the control rod a discrete incremental distance or "step;" hence, such movement is commonly referred to as stepping of the control rods. There are typically 231 steps between the fully withdrawn position and the fully inserted position of the control rods. For example, 0 steps indicate the fully inserted position, and 231 steps indicate the fully withdrawn position. This type of mechanism is illustrated and described in U.S. Pat. Nos. to Frisch (3,158,766) and Dewesse (3,992,255) which are assigned to the assignee of the present invention.
This magnetic jack type of control rod drive mechanism includes three electromagnetic coils and armatures or plungers which are operated to raise and lower a drive rod shaft and thereby the control rod cluster assembly. The three coils are mounted about and outside of the pressure housing. Two of the coils actuate respective plungers of movable and stationary grippers contained within the housing. The third coil actuates a lift plunger connected to the movable gripper. Actuation of the movable and stationary plungers, in turn, operate sets of circumferentially spaced latches which grip the drive rod shaft having multiple axially-spaced circumferential grooves. The stationary gripper latches are actuated to hold the drive shaft in a desired axial position. The movable gripper latches are actuated to raise and lower the drive rod shaft. Each jacking or stepping movement of the control rod drive mechanism moves the drive rod shaft 5/8 inch (1.58 cm) The jacking or stepping movement is thus accomplished by the operation of the three sets of axially spaced electromagnetic coils to actuate the corresponding stationary, movable and lift plungers so as to alternately and sequentially grip, move and release the control rod drive shaft of the respective mechanism.
A number of indicators have been used in the past to determine control rod position. One such indicator is an analog indicator. This analog indicator includes a plurality of layered, wound coils concentrically arranged in a stack and supported by a nonmagnetic stainless steel tubular substructure that is slid over a nonmagnetic rod travel housing. The coils are arranged alternately as primary and secondary coils, with all the primary coils connected in series and all the secondary coils connected in series. The coils form, in effect, a long linear voltage transformer distributed over the height of the travel housing such that the coupling from primary to secondary is affected by the extent to which the magnetic drive rod penetrates the coil stack. Rod position is determined by applying a constant sinusoidal excitation current to the primary and measuring the voltage induced across the secondary. The magnitude of the induced secondary voltage corresponds to rod position. The secondary output voltage varies substantially linearly with the position of control rod, as is well known in the art. This secondary voltage is processed by instrumentation, which is well known in the art, and displayed on a control panel.
Although the present device for detecting control rod position is satisfactory, it is not without drawbacks. When the control rods are near the fully withdrawn position (i.e., 231 steps), the output of the transformer varies substantially non-linearly with the position of the control rods, which obviously creates an error in the indication of the position of the control rod.
Consequently, a need exists for a method and apparatus for compensating a rod position indication system for non-linearity when the control rods are substantially withdrawn.